Because of its importance to humans, facial expression recognition is also a major topic in the field of machine learning. If machines would be able to interpret human facial expression correctly - and possibly make appropriate facial expressions in return - human-to-machine interaction would become more natural and efficient. Recognition of facial expression by machines is called automatic facial expression recognition, or automatic FER. Automatic FER has come a long way, from hand-crafted approaches to the current end-to-end deep learning models that locate and recognize facial expressions [1]}. Nonetheless, there still is a long way to go: current algorithms are good enough at recognizing laboratory-controlled facial expression images, but they struggle to recognize expressions from naturalistic images [2]}, [3]}.
